1. Field of the Invention
This invention relates to gas flow measuring devices which operate on the principle of measuring the time displacement of a soap bubble between two points on a flow tube. The displacement in time is converted to a measurement of flow.
2. Description of Prior Art
It is customary to sample the atmosphere of a work place or hostile environment to detect for the presence of a specific constituent or toxic contaminant and to measure its concentration. This is done by drawing a known volume of air through an air sampling system containing one or more filters or sorbent tubes which may be directly or subsequently analyzed to quantify the level of the constituent or contaminant in the air sample. The accuracy in calculating the concentration of any specific constituent in the air sample depends upon the accuracy in the measurement of the rate of air flow through the air sampling equipment.
One method for measuring air flow is to time the displacement of a film of soap solution between two points in a flow tube. At present, the position of the film of soap is optically detected using "LED" photodetectors as sensing elements. Two sets of photodetectors are required with each set aligned on opposite sides of the flow tube. The two sets are spaced a predetermined distance apart to define a measured distance. When the film of soap crosses a beam of light transmitted between the first set of photodetectors, the light beam is interrupted and a timing operation is initiated. The timing operation is terminated when the soap film crosses the light beam transmitted between the second set of photodetectors. Flow measurement is then calculated from a measurement of the transit time of the soap film between the two sets of photodetectors. This calculation can readily be accomplished automatically and displayed visually.
The accuracy of the flow measurement using an optical method of detection is dependent upon how accurately the photodetectors are aligned. The alignment of each set of photodetectors is a relatively complex and time consuming operation which adds considerably to the cost of manufacture of the flowmeter. In addition, since the flowmeter is designed for portable use, the additional power required by the photodetectors not only adds to the overall power requirement for the flowmeter, but represents a considerable power drain. Conversely, a flowmeter which does not require photodetectors to detect the soap bubble can be made smaller in size and more efficient in the use of power. This is particularly significant for portable operation.